Retardation optical element, process of producing the same, and liquid crystal display

ABSTRACT

The present invention provides a retardation optical element comprising a negative C plate (retardation layer) and an A plate (retardation layer), never causing the appearance of bright-and-dark patterns on the displayed image even when placed between a liquid crystal cell and a polarizer, thus being capable of effectively preventing lowering of display quality. A retardation optical element  10  comprises: a C plate type retardation layer  12  that has a cholesteric structure with liquid crystalline molecules in planar orientation and functions as a negative C plate; and an A plate type retardation layer  14  that has a nematic structure and functions as an A plate. The C plate type retardation layer  12  and the A plate type retardation layer  14  are laminated adjacently to each other, and the directions of the directors Cb of liquid crystalline molecules on the surface  12 B, on the A plate type retardation layer  14  side, of the C plate type retardation layer  12  are substantially the same as the directions of the directors Na of liquid crystalline molecules on the surface  14 A, on the C plate type retardation layer  12  side, of the A plate type retardation layer  14 . Further, the C plate type retardation layer  12  is made to have a helical structure with a helical pitch so adjusted that the C plate type retardation layer  12  selectively reflects, owing to its structure, light whose wavelength falls in a wave range that is different from the wave range of incident light.

FIELD OF THE INVENTION

The present invention relates to a retardation optical element for usein a liquid crystal display or the like, especially a retardationoptical element that comprises a retardation layer functioning as anegative C plate and a retardation layer functioning as an A plate. Byusing this retardation optical element, of the light incident on and/oremerging from a liquid crystal cell, the light emerging slantingly fromthe liquid crystal cell in the direction deviating from the normal tothe liquid crystal cell is compensated for the state of polarization.The present invention also relates to a process of producing theretardation optical element, and to a liquid crystal display comprisingthe retardation optical element.

BACKGROUND ART

FIG. 9 is a diagrammatic, exploded perspective view of a conventional,standard liquid crystal display.

As shown in FIG. 9, the conventional liquid crystal display 100comprises a polarizer 102A on the incident side, a polarizer 102B on theemergent side, and a liquid crystal cell 104.

Of these components, the polarizers 102A and 102B are so made that theyselectively transmit only linearly polarized light having the plane ofvibration in a predetermined direction, and are arranged in the crossnicol disposition so that the direction of vibration of the linearlypolarized light transmitted by the polarizer 102A is perpendicular tothat of vibration of the linearly polarized light transmitted by thepolarizer 102B. The liquid crystal cell 104 comprises a large number ofcells corresponding to pixels, and is placed between the polarizers 102Aand 102B.

The case where the liquid crystal cell 104 in the above-described liquidcrystal display 100 is of VA (Vertical Alignment) mode, in which anematic liquid crystal having negative dielectric anisotropy is sealedin a liquid crystal cell, is now taken as an example (in the figure, adotted line diagrammatically indicates the direction of the director ofthe liquid crystal). Linearly polarized light that has passed throughthe polarizer 102A on the incident side passes, without undergoing phaseshift, through those cells in the liquid crystal cell 104 that are inthe non-driven state, and is blocked by the polarizer 102B on theemergent side. On the contrary, the linearly polarized light undergoesphase shift while it passes through those cells in the liquid crystalcell 104 that are in the driven state, and the light in an amountcorresponding to the amount of this phase shift passes through andemerges from the polarizer 102B on the emergent side. It is thereforepossible to display the desired image on the emergent-side polarizer102B side by properly, individually controlling the driving voltage thatis applied to each cell in the liquid crystal cell 104. The liquidcrystal display 100 is not limited to the above embodiment in whichlight is transmitted and blocked in the above-described manner, andthere is also a liquid crystal display so constructed that lightemerging from those cells in the liquid crystal cell 104 that are in thenon-driven state passes through and emerges from the polarizer 102B onthe emergent side, but that light emerging from those cells that are inthe driven state is blocked by the polarizer 102B on the emergent side.

Consideration will now be given to the situation that linearly polarizedlight passes through the non-driven-state cells in the above-describedliquid crystal cell 104 of VA mode. The liquid crystal cell 104 isbirefringent, and its refractive index in the direction of thickness isdifferent from its refractive indices in the direction of plane.Therefore, of the linearly polarized light that has passed through thepolarizer 102A on the incident side, the light that has entered theliquid crystal cell 104 along the normal to the liquid crystal cell 104passes through the liquid crystal cell 104 without undergoing phaseshift, but the light that has slantingly entered the liquid crystal cell104 in the direction deviating from the normal to the liquid crystalcell 104 undergoes phase shift while it passes through the liquidcrystal cell 104 and becomes elliptically polarized light. The cause ofthis phenomenon is that those liquid crystalline molecules, which arevertically oriented in the liquid crystal cell 104 when the cells in theliquid crystal cell 104 of VA mode are in the non-driven state, functionas a positive C plate. It is noted that the amount of phase shift thatoccurs for light passing through the liquid crystal cell 104(transmitted light) is affected also by the birefringence of the liquidcrystalline molecules sealed in the liquid crystal cell 104, thethickness of the liquid crystal cell 104, the wavelength of thetransmitted light, and so on.

Owing to the above-described phenomenon, even when the cells in theliquid crystal cell 104 are in the non-driven state and linearlypolarized light should pass through the liquid crystal cell 104 as it isand should be blocked by the polarizer 102B on the emergent side, a partof the light emerging slantingly from the liquid crystal cell 104 in thedirection deviating from the normal to the liquid crystal cell 104 is toleak from the polarizer 102B on the emergent side.

For this reason, the above-described conventional liquid crystal display100 has the problem that the display quality at the time when thedisplayed image is viewed obliquely from a position not on the normal tothe liquid crystal cell 104 tends to be poorer than the display qualityat the time when this image is viewed from a position right in front ofthe display (viewing angle dependency problem).

In order to improve the viewing angle dependency of the above-describedconventional liquid crystal display 100, a variety of techniques havebeen developed up to now. Such a liquid crystal display as is describedin Japanese Laid-Open Patent Publication No. 67219/1991, for example,has been known as one of these techniques. This liquid crystal displayuses a retardation optical element comprising a retardation layer havinga cholesteric structure (a birefringent retardation layer), where theretardation optical element is placed between a liquid-crystal cell anda polarizer in order to provide optical compensation.

In the retardation optical element having a cholesteric structure, theselective reflection wavelength given by the equation λ=nav·p (p: thehelical pitch in the helical structure consisting of liquid crystallinemolecules, nav: the mean refractive index on the plane perpendicular tothe helical axis) is adjusted so that it is either shorter or longerthan the wavelength of transmitted light, as described in JapaneseLaid-Open Patent Publication No. 322223/1992, for example.

In the retardation optical element described above, linearly polarizedlight that has slantingly entered the retardation layer in the directiondeviating from the normal to the retardation layer undergoes phaseshift, while it passes through the retardation layer, to becomeelliptically polarized light, as in the case of the above-describedliquid crystal cell. The cause of this phenomenon is that thecholesteric structure functions as a negative C plate. The amount ofphase shift that occurs for light passing through the retardation layer(transmitted light) is affected also by the birefringence of the liquidcrystalline molecules in the retardation layer, the thickness of theretardation layer, the wavelength of the transmitted light, and so on.

It is therefore possible to significantly improve the viewing angledependency of conventional liquid crystal displays by using theabove-described retardation optical element, if the retardation layercontained in the retardation optical element is properly designed sothat the phase shift that occurs in a liquid crystal cell of VA modefunctioning as a positive C plate and the phase shift that occurs in theretardation layer functioning as a negative C plate cancel each other.

The viewing angle dependency of liquid crystal displays can be improvedmore significantly by using a retardation layer that functions as anegative C plate (i.e., a retardation layer in which the relationshipsamong its refractive indices Nx and Ny in the direction of plane and itsrefractive index Nz in the direction of thickness are Nx=Ny>Nz) and aretardation layer that functions as an A plate (i.e., a retardationlayer in which the relationships among its refractive indices Nx and Nyin the direction of plane and its refractive index Nz in the directionof thickness are Nx>Ny=Nz) in combination, as described in JapaneseLaid-Open Patent Publication No. 258605/1999, for example.

However, it has been found that, in the case where the above-describedconventional retardation optical element (a retardation layer having acholesteric structure, functioning as a negative C plate) is placedbetween a liquid crystal cell and a polarizer, although viewing angledependency can be improved, bright-and-dark patterns, etc. can appear onthe displayed image to greatly lower the display quality. In particular,it has been found that the display quality lowers drastically when aretardation layer functioning as a negative C plate and a retardationlayer functioning as an A plate are, as described above, used incombination for a retardation optical element.

Conducting experiments and computer-aided simulations, the inventor hasearnest studies in order to find the reason why such a retardationoptical element (comprising a retardation layer functioning as anegative C plate and a retardation layer functioning as an A plate)causes the appearance of bright-and-dark patterns, etc. As a result, theinventor has finally found that this phenomenon is partly attributed tothe directions of the directors of liquid crystalline molecules on thesurfaces of the retardation layers.

Regarding a circularly-polarized-light-extracting optical elementcomprising one or more cholesteric liquid crystal layers, the inventorhas already made a variety of proposals on the directions of thedirectors of liquid crystalline molecules on the surfaces of the liquidcrystal layer(s) and also on the directions of the directors of liquidcrystalline molecules in the vicinity of the interface between twoneighboring liquid crystal layers (Japanese Laid-Open Patent PublicationNo. 189124/2002, and Japanese Patent Application No. 60392/2001(Japanese Laid-Open Patent Publication No. 258053/2002)). However, theseproposals are only for a circularly-polarized-light-extracting opticalelement comprising a single, cholesteric liquid crystal layer or aplurality of cholesteric liquid crystal layers that are laminated toeach other, and a construction suitable for such a retardation opticalelement (comprising a retardation layer functioning as a negative Cplate and a retardation layer functioning as an A plate) as is describedabove has not yet been completely made clear.

SUMMARY OF THE INVENTION

The present invention has been accomplished under these circumstances.Objects of the present invention are therefore to provide a retardationoptical element comprising a retardation layer that functions as anegative C plate and a retardation layer that functions as an A plate,never causing the appearance of bright-and-dark patterns, etc. on thedisplayed image even when placed between a liquid crystal cell and apolarizer, thus being capable of effectively preventing lowering ofdisplay quality; a process of producing the retardation optical element;and a liquid crystal display comprising the retardation optical element.

The present invention provides, as a first aspect for fulfilling theobject of the invention, a retardation optical element comprising: a Cplate type retardation layer that has a cholesteric structure withliquid crystalline molecules in planar orientation and functions as anegative C plate, the helical pitch in the structure being so adjustedthat the C plate type retardation layer selectively reflects, owing toits structure, light whose wavelength falls in a wave range that isdifferent from the wave range of incident light; and an A plate typeretardation layer that has a nematic structure, functions as an A plate,and is laminated adjacently to the C plate retardation layer, whereinthe directions of the directors of liquid crystalline molecules on oneof the two main opposite surfaces of the C plate type retardation layer,situated on the side of the A plate type retardation layer, aresubstantially the same as the directions of the directors of liquidcrystalline molecules on one of the two main opposite surfaces of the Aplate type retardation layer, situated on the side of the C plate typeretardation layer.

In the above-described first aspect for fulfilling the object of thepresent invention, it is preferable that the directions of the directorsof liquid crystalline molecules on one of the two main opposite surfacesof the C plate type retardation layer, situated on the side of the Aplate type retardation layer, be substantially parallel to thedirections of the directors of liquid crystalline molecules on the othersurface of the C plate type retardation layer, situated on the sideapart from the A plate type retardation layer.

Further, in the above-described first aspect for fulfilling the objectof the invention, it is preferable that the directions of the directorsof liquid crystalline molecules on the surface, on the side apart fromthe A plate type retardation layer, of the C plate type retardationlayer be substantially parallel to the directions of the directors ofliquid crystalline molecules on the surface, on the side apart from theC plate type retardation layer, of the A plate type retardation layer.

Furthermore, in the above-described first aspect for fulfilling theobject of the invention, it is preferable that the C plate typeretardation layer has a helical structure with a pitch numbersubstantially equal to (0.5×integer) between its two main oppositesurfaces, that is, the surface on the side of the A plate typeretardation layer and the surface on the side apart from the A platetype retardation layer.

Furthermore, in the above-described first aspect for fulfilling theobject of the invention, it is preferable that the C plate typeretardation layer has a structure of a chiral nematic liquid crystalthat is solidified by means of three-dimensional cross-linking or of apolymeric, cholesteric liquid crystal that is solidified into a glassystate.

Furthermore, in the above-described first aspect for fulfilling theobject of the invention, it is preferable that the A plate typeretardation layer has a structure of a nematic liquid crystal that issolidified by means of three-dimensional cross-linking or of apolymeric, nematic liquid crystal that is solidified into a glassystate.

The present invention provides, as a second aspect for fulfilling theobject of the invention, a process of producing a retardation opticalelement, comprising: applying a first liquid crystal to an alignmentlayer whose surface has alignment regulation power in substantially onedirection on its entire surface, the first liquid crystal having acholesteric regularity and being so prepared that the liquid crystalwhen solidified selectively reflects light whose wavelength falls in awave range that is different from the wave range of incident light;solidifying the first liquid crystal applied, with the directions of thedirectors of liquid crystalline molecules on a surface of the firstliquid crystal being regulated by the alignment regulation power of thealignment layer, thereby forming a C plate type retardation layer thatfunctions as a negative C plate; applying a second liquid crystaldirectly to the C plate type retardation layer formed, the second liquidcrystal having a nematic regularity; and solidifying the second liquidcrystal applied, with the directions of the directors of liquidcrystalline molecules on the surface, situated on the side of the Cplate type retardation layer, of the second liquid crystal beingregulated by the alignment regulation power of the surface of the Cplate type retardation layer, thereby forming an A plate typeretardation layer that functions as an A plate.

In the above-described second aspect for fulfilling the object of thepresent invention, it is preferable that the first liquid crystal be aliquid crystal comprising at least one of polymerizable cholestericmonomers and oligomers; that the first liquid crystal be solidified bymeans of three-dimensional cross-linking, with the directions of thedirectors of liquid crystalline molecules on the surface of the firstliquid crystal being regulated by the alignment regulation power of thealignment layer; that the second liquid crystal be a liquid crystalcomprising at least one of polymerizable nematic monomers and oligomers;and that the second liquid crystal be solidified by means ofthree-dimensional cross-linking, with the directions of the directors ofliquid crystalline molecules on the surface, situated on the side of theC plate type retardation layer, of the second liquid crystal beingregulated by the alignment regulation power of the surface of the Cplate type retardation layer.

Further, in the second aspect for fulfilling the object of the presentinvention, it is preferable that the first liquid crystal be a liquidcrystal comprising a cholesteric liquid crystalline polymer; that thefirst liquid crystal be solidified into a glassy state by cooling, withthe directions of the directors of liquid crystalline molecules on thesurface of the first liquid crystal being regulated by the alignmentregulation power of the alignment layer; that the second liquid crystalbe a liquid crystal comprising a nematic liquid crystalline polymer; andthat the second liquid crystal be solidified into a glassy state bycooling, with the directions of the directors of liquid crystallinemolecules on the surface, situated on the side of the C plate typeretardation layer, of the second liquid crystal being regulated by thealignment regulation power of the surface of the C plate typeretardation layer.

In the above-described second aspect for fulfilling the object of theinvention, it is preferable to adjust the coating thickness of the firstliquid crystal so that the directions of the directors of liquidcrystalline molecules on the two main opposite surfaces of the C platetype retardation layer are substantially parallel to each other.

Furthermore, in the above-described second aspect for fulfilling theobject of the invention, it is preferable to bring another alignmentlayer into contact with the surface, situated on the side apart from thesurface of the above-described alignment layer, of the C plate typeretardation layer, so that the liquid crystal is solidified with thedirections of the directors of liquid crystalline molecules on the twomain opposite surfaces of the C plate type retardation layer beingregulated.

Furthermore, in the above-described second aspect for fulfilling theobject of the invention, it is preferable to bring another alignmentlayer into contact with the surface, situated on the side apart from thesurface of the C plate type retardation layer, of the A plate typeretardation layer, so that the second liquid crystal is solidified withthe directions of the directors of liquid crystalline molecules on thetwo main opposite surfaces of the A plate type retardation layer beingregulated.

The present invention provides, as a third aspect for fulfilling theobject of the invention, a process of producing a retardation opticalelement, comprising: applying a first liquid crystal to an alignmentlayer whose surface has alignment regulation power in substantially onedirection on its entire surface, the first liquid crystal having anematic regularity; solidifying the first liquid crystal applied, withthe directions of the directors of liquid crystalline molecules on asurface of the first liquid crystal being regulated by the alignmentregulation power of the alignment layer, thereby forming an A plate typeretardation layer that functions as an A plate; applying a second liquidcrystal directly to the A plate type retardation layer formed, thesecond liquid crystal having a cholesteric regularity and being soprepared that the liquid crystal when solidified selectively reflectslight whose wavelength falls in a wave range that is different from thewave range of incident light; and solidifying the second liquid crystalapplied, with the directions of the directors of liquid crystallinemolecules on the surface, situated on the side of the A plate typeretardation layer, of the second liquid crystal being regulated by thealignment regulation power of the surface of the A plate typeretardation layer, thereby forming a C plate type retardation layer thatfunctions as a negative C plate.

In the above-described third aspect for fulfilling the object of thepresent invention, it is preferable that the first liquid crystal be aliquid crystal comprising at least one of polymerizable nematic monomersand oligomers; that the first liquid crystal be solidified by means ofthree-dimensional cross-linking, with the directions of the directors ofliquid crystalline molecules on the surface of the first liquid crystalbeing regulated by the alignment regulation power of the alignmentlayer; that the second liquid crystal be a liquid crystal comprising atleast one of polymerizable cholesteric monomers and oligomers; and thatthe second liquid crystal be solidified by means of three-dimensionalcross-linking, with the directions of the directors of liquidcrystalline molecules on the surface, situated on the side of the Aplate type retardation layer, of the second liquid crystal beingregulated by the alignment regulation power of the surface of the Aplate type retardation layer.

Further, in the third aspect for fulfilling the object of the presentinvention, it is preferable that the first liquid crystal be a liquidcrystal comprising a nematic liquid crystalline polymer; that the firstliquid crystal be solidified into a glassy state by cooling, with thedirections of the directors of liquid crystalline molecules on thesurface of the first liquid crystal being regulated by the alignmentregulation power of the alignment layer; that the second liquid crystalbe a liquid crystal comprising a cholesteric liquid crystalline polymer;and that the second liquid crystal be solidified into a glassy state bycooling, with the directions of the directors of liquid crystallinemolecules on the surface, situated on the side of the A plate typeretardation layer, of the second liquid crystal being regulated by thealignment regulation power of the surface of the A plate typeretardation layer.

In the above-described third aspect for fulfilling the object of theinvention, it is preferable to adjust the coating thickness of thesecond liquid crystal so that the directions of the directors of liquidcrystalline molecules on the two main opposite surfaces of the C platetype retardation layer are substantially parallel to each other.

Furthermore, in the above-described third aspect for fulfilling theobject of the invention, it is preferable to bring another alignmentlayer into contact with the surface, situated on the side apart from thesurface of the A plate type retardation layer, of the C plate typeretardation layer, so that the second liquid crystal is solidified withthe directions of the directors of liquid crystalline molecules on thetwo main opposite surfaces of the C plate type retardation layer beingregulated.

Furthermore, in the above-described third aspect for fulfilling theobject of the invention, it is preferable to bring another alignmentlayer into contact with the surface, situated on the side apart from thesurface of the above-described alignment layer, of the A plate typeretardation layer, so that the second liquid crystal is solidified withthe directions of the directors of liquid crystalline molecules on thetwo main opposite surfaces of the A plate type retardation layer beingregulated.

The present invention provides, as a fourth aspect for fulfilling theobject of the invention, a liquid crystal display comprising: a liquidcrystal cell; a pair of polarizers so arranged that the liquid crystalcell is sandwiched therebetween; and a retardation optical elementaccording to the above-described first aspect for fulfilling the objectof the invention, placed between the liquid crystal cell and at leastone of a pair of the polarizers, wherein, of the light in apredetermined state of polarization, incident on and/or emerging fromthe liquid crystal cell, the light emerging slantingly in the directiondeviating from the normal to the liquid crystal cell is compensated bythe retardation optical element for the state of polarization.

According to the retardation optical element of the first aspect forfulfilling the object of the invention, in the retardation opticalelement comprising a C plate type retardation layer that has acholesteric structure with liquid crystalline molecules in planarorientation and functions as a negative C plate, and an A plate typeretardation layer that has a nematic structure, functions as an A plate,and is laminated adjacently to the C plate type retardation layer, thehelical pitch in the structure of the C plate type retardation layer isso adjusted that the C plate type retardation layer selectivelyreflects, owing to its structure, light whose wavelength falls in a waverange that is different from the wave range of incident light, and thedirections of the directors of liquid crystalline molecules on the twoneighboring surfaces of the C plate type retardation layer and the Aplate type retardation layer are made substantially the same. Therefore,the retardation optical element never causes the appearance ofbright-and-dark patterns, etc. on the displayed image even when placedbetween a liquid crystal cell and a polarizer and can thus effectivelyprevent lowering of display quality.

In the retardation optical element according to the first aspect forfulfilling the object of the invention, by making the directions of thedirectors of liquid crystalline molecules on the two main oppositesurfaces of the C plate type retardation layer be substantially parallelto each other, it is possible to more effectively prevent the appearanceof bright-and-dark patterns, etc., and is thus possible to furtherprevent lowering of display quality.

Further, in the retardation optical element according to the firstaspect for fulfilling the object of the invention, by making,substantially parallel to each other, the directions of the directors ofliquid crystalline molecules on the two main opposite surfaces, not incontact with each other, of the C plate type retardation layer and the Aplate type retardation layer that are laminated adjacently to eachother, it is possible to more effectively prevent the appearance ofbright-and-dark patterns, etc.

Furthermore, in the retardation optical element according to the firstaspect for fulfilling the object of the invention, by forming the Cplate type retardation layer to have a helical structure with a pitchnumber substantially equal to (0.5×integer) between its two mainopposite surfaces, it is possible to make the directions of thedirectors of liquid crystalline molecules on the two main oppositesurfaces of the C plate type retardation layer be the same with highaccuracy. By this, it is possible to prevent the appearance ofbright-and-dark patterns, etc. more effectively and is thus possible tofurther prevent lowering of display quality.

According to the process of producing a retardation optical element ofthe second aspect for fulfilling the object of the present invention, afirst liquid crystal having a cholesteric regularity is applied to analignment layer whose surface has alignment regulation power insubstantially one direction on its entire surface, thereby forming a Cplate type retardation layer that functions as a negative C plate, and asecond liquid crystal having a nematic regularity is then applieddirectly to this C plate type retardation layer to form an A plate typeretardation layer that functions as an A plate. Thus, it is possible toeasily produce a retardation optical element comprising a retardationlayer that functions as a negative C plate and a retardation layer thatfunctions as an A plate, never causing the appearance of bright-and-darkpatterns, etc. on the displayed image, thus being capable of effectivelypreventing lowering of display quality.

According to the process of producing a retardation optical element ofthe third aspect for fulfilling the object of the invention, a firstliquid crystal having a nematic regularity is applied to an alignmentlayer whose surface has alignment regulation power in substantially onedirection on its entire surface, thereby forming an A plate typeretardation layer that functions as an A plate, and a second liquidcrystal having a cholesteric regularity is then applied directly to thisA plate type retardation layer to form a C plate type retardation layerthat functions as a negative C plate. Thus, it is possible to easilyproduce a retardation optical element comprising a retardation layerthat functions as a negative C plate and a retardation layer thatfunctions as an A plate, never causing the appearance of bright-and-darkpatterns, etc. on the displayed image, thus being capable of effectivelypreventing lowering of display quality.

According to the fourth aspect for fulfilling the object of the presentinvention, a retardation optical element is placed between a liquidcrystal cell and a polarizer in a liquid crystal display, whereby, ofthe light in a predetermined state of polarization, incident on and/oremerging from the liquid crystal cell, the light slantingly emerging inthe direction deviating from the normal to the liquid crystal cell iscompensated by the retardation optical element for the state ofpolarization. Thus, it is possible to prevent the appearance ofbright-and-dark patterns, etc. on the liquid crystal display and, at thesame time, improve contrast. Lowering of display quality can thus beavoided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an enlarged perspective view diagrammatically showing a partof a retardation optical element according to an embodiment of thepresent invention;

FIG. 2 is an enlarged perspective view diagrammatically showing a partof a modification of the retardation optical element according to anembodiment of the present invention;

FIG. 3 is a diagrammatic view showing the relationship between thehelical pitch in the cholesteric, helical structure consisting of liquidcrystalline molecules and the directors of liquid crystalline moleculeson the surfaces of a retardation layer;

FIG. 4 is a diagrammatic cross-sectional view for explaining a firstprocess of producing a retardation optical element according to anembodiment of the present invention;

FIG. 5 is a diagrammatic cross-sectional view for explaining amodification of the first process of producing a retardation opticalelement according to an embodiment of the present invention;

FIG. 6 is a diagrammatic cross-sectional view for explaining a secondprocess of producing a retardation optical element according to anembodiment of the present invention;

FIG. 7 is a diagrammatic, exploded perspective view showing a liquidcrystal display comprising a retardation optical element according to anembodiment of the present invention;

FIG. 8 is a diagrammatic, exploded perspective view showing adisposition of a retardation optical element and polarizers at the timewhen the retardation optical element sandwiched between the polarizersis observed; and

FIG. 9 is a diagrammatic, exploded perspective view showing aconventional liquid crystal display.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of the present invention will be described hereinafter withreference to the accompanying drawings.

A retardation optical element according to an embodiment is firstlydescribed with reference to FIG. 1.

As shown in FIG. 1, a retardation optical element 10 comprises: a Cplate type retardation layer 12 having a cholesteric structure withliquid crystalline molecules in planar orientation; and an A plateretardation layer 14 having a nematic structure, laminated adjacently tothe C plate type retardation layer 12.

Of the two retardation layers, the C plate type retardation layer 12 hastwo main opposite surfaces (surfaces with larger areas) 12A and 12B thatare perpendicular to the direction of thickness (the direction of thenormal 15). The C plate type retardation layer 12 is anisotropic, thatis, birefringent, owing to its cholesteric structure, and its refractiveindex in the direction of thickness is different from its refractiveindices in the direction of plane, so that this retardation layer 12functions as a negative C plate. Namely, in the three-dimensionalrectangular coordinates, when the refractive indices of the C plate typeretardation layer 12 in the direction of plane are indicated by Nx andNy and the refractive index in the direction of thickness, by Nz, therelationships among these refractive indices are Nx=Ny>Nz.

Further, the A plate type retardation layer 14 has two main oppositesurfaces (surfaces with larger areas) 14A and 14B that are perpendicularto the direction of thickness (the direction of the normal 15). The Aplate type retardation layer 14 is anisotropic, that is, birefringent,owing to its nematic structure, and its refractive indices in thedirection of plane are different from each other, so that thisretardation layer 14 functions as a (positive) A plate. Namely, in thethree-dimensional rectangular coordinates, when the refractive indicesof the A plate type retardation layer 14 in the direction of plane areindicated by Nx and Ny and the refractive index in the direction ofthickness, by Nz, the relationships among these refractive indices areNx>Ny=Nz.

The directions of the directors Cb of liquid crystalline molecules onthe surface 12B, situated on the A plate type retardation layer 14 side,of the C plate type retardation layer 12 are substantially the same asthe directions of the directors Na of liquid crystalline molecules onthe surface 14A, situated on the C plate type retardation layer 12 side,of the A plate type retardation layer 14. The dispersion in thedirections of the directors Cb of liquid crystalline molecules on thesurface 12B of the C plate type retardation layer 12 and of thedirectors Na of liquid crystalline molecules on the surface 14A of the Aplate type retardation layer 14 are within ±10°, preferably within ±5°,more preferably within ±1°.

The expression “substantially the same” as used herein encompasses thecase where the directions of the directors of liquid crystallinemolecules are different by an angle of nearly 180°, that is, the head ofa liquid crystalline molecule and the tail of another one are in thesame direction. This is because, in many cases, the head of a liquidcrystalline molecule is optically indistinguishable from its tail. Thesame is true for the cases that will be described later (the case wherethe directions of the directors Ca and Cb of liquid crystallinemolecules on the two main opposite surfaces 12A and 12B of the C platetype retardation layer 12 are “substantially parallel” to each other,and the case where the directions of the directors Ca and Nb of liquidcrystalline molecules on the two surfaces 12A and 14B, not in contactwith each other, of the C plate type retardation 12 and the A plate typeretardation layer 14 are “substantially parallel” to each other).

The term “liquid crystalline molecules” is usually used to indicatethose molecules that have both the fluidity of liquid and the anisotropyof crystal. In this specification, however, this term “liquidcrystalline molecules” is, for convenience' sake, used also to indicatethose molecules that have been solidified with the anisotropy which themolecules possessed when they were in the fluid state retained. Examplesof methods for solidifying molecules while retaining the anisotropywhich the molecules possessed when they were in the fluid state includea method in which liquid crystalline molecules having polymerizablegroups (polymerizable monomer or oligomer molecules) are cross-linked,and a method in which a polymeric liquid crystal (liquid crystallinepolymer) is cooled to a temperature below its glass transitiontemperature.

It is preferable that the directions of the directors Ca of liquidcrystalline molecules on one surface 12A of the two main oppositesurfaces of the C plate type retardation layer 12 be substantially thesame and that the directions of the directors Cb of liquid crystallinemolecules on the other surface 12B of the C plate type retardation layer12 be also substantially the same. It is herein preferable that thedispersion in the directions of the directors Ca and Cb of liquidcrystalline molecules on one surface 12A and the other surface 12B ofthe C plate type retardation layer 12 be within ±10°, preferably within±5°, more preferably within ±1°.

Whether the directions of the directors Ca and Cb of the liquidcrystalline molecules on the surfaces 12A and 12B of the C plate typeretardation layer 12 are substantially the same or not can be judgedfrom the observation of the cross section of the C plate typeretardation layer 12, using a transmission electron microscope.Specifically, when the cross section of the C plate type retardationlayer 12 solidified with its liquid crystal structure kept cholestericis examined using a transmission electron microscope, bright-and-darkpatterns corresponding to the pitches of the molecular helixes,characteristic of a cholesteric liquid crystal structure, are observed.Therefore, if the bright-and-dark patterns that appear on each surface12A or 12B are seen almost uniformly over the plane in terms ofbrightness, it can be judged that the directions of the directors ofliquid crystalline molecules on this plane are substantially the same.

In the retardation optical element 10 shown in FIG. 1, the C plate typeretardation layer 12 having a cholesteric structure, functioning as anegative C plate, is birefringent, and its refractive index in thedirection of thickness is different from its refractive indices in thedirection of plane. Therefore, although linearly polarized light thatenters the C plate type retardation layer 12 along the normal 15 to itis transmitted without undergoing phase shift, linearly polarized lightthat slantingly enters the C plate type retardation layer 12 in thedirection deviating from the normal 15 to it undergoes phase shift whilethe light passes through the C plate type retardation layer 12 andbecomes elliptically polarized light. In contrast with this, ifelliptically polarized light slantingly enters the C plate typeretardation layer 12 in the direction deviating from the normal 15 toit, the C plate type retardation layer 12 can also convert thiselliptically polarized light into linearly polarized light.

On the other hand, also the A plate type retardation layer 14 having anematic structure, functioning as an A plate, is birefringent, providedthat its refractive indices in the direction of plane are different fromeach other. Namely, even in the direction along the surfaces 14A and14B, the refractive index in the direction of the director Na or Nb isdifferent from that in the direction perpendicular to the director Na orNb. It is noted that the refractive index in the direction perpendicularto the director Na or Nb is equal to the refractive index in thedirection of thickness.

For this reason, by making a retardation optical element by thecombination use of these two retardation layers (the C plate typeretardation layer 12 and the A plate type retardation layer 14), thetypes of birefringence of the retardation layers being different indirection, it is possible to cause phase shift to both light passingthrough the retardation optical element in the direction of the normal15 and light passing through the retardation optical element in thedirection deviating from the normal 15, and is thus possible to providevarious types of optical compensation.

In this retardation optical element, the C plate type retardation layer12 and the A plate type retardation layer 14 are laminated adjacently toeach other, and the directions of the directors Cb of liquid crystallinemolecules on the surface 12B of the C plate type retardation layer 12and the directions of the directors Na of liquid crystalline moleculeson the surface 14A of the A plate type retardation layer 14, the surface12B and the surface 14A being in contact with each other, aresubstantially the same. Thus, the retardation optical element nevercauses the appearance of bright-and-dark patterns, etc. on the displayedimage even when placed between a liquid crystal cell and a polarizer ina liquid crystal display. The retardation optical element can thuseffectively prevent lowering of display quality.

In the C plate type retardation layer 12, the helical pitch in itsstructure is so adjusted that the C plate type retardation layer 12selectively reflects, owing to its cholesteric structure, light whosewavelength falls in a wave range that is different from the wave rangeof incident light (the wave range in which the wavelength of light to beselectively reflected falls is either shorter or longer than thewavelength of incident light). If the helical pitch is so adjusted, evenwhen the incident light is, for example, visible light, the incidentlight (visible light) is never reflected by means of selectivereflection that occurs owing to the helical structure consisting ofliquid crystalline molecules, and such problems as coloring never occur.

A brief explanation for selective reflection, a phenomenon in the Cplate type retardation layer 12 having a cholesteric structure, will begiven below.

A cholesteric structure has the rotated-light-selecting property(polarized-light-separating property) of separating a componentoptically rotated (circularly polarized) in one direction from acomponent optically rotated in the opposite direction, according to theplanar orientation of a liquid crystal.

This phenomenon is known as circular dichroism. If the direction ofrotation of liquid crystalline molecules in the helical structure isproperly selected, a component circularly polarized in the samedirection as this direction of rotation is selectively reflected.

In this case, the scattering of polarized light is maximized (theselective reflection peaks) at the wavelength λ0 given by the followingequation (1):λ0=nav·p,  (1)wherein p is the helical pitch in the helical structure consisting ofliquid crystalline molecules, and nav is the mean refractive index on aplane perpendicular to the helical axis.

On the other hand, the width Δλ of the wave range in which thewavelength of light to be selectively reflected falls is given by thefollowing equation (2):Δλ=Δn·p,  (2)wherein Δn is the value of birefringence which is defined as adifference between the index of refraction for normal light and that ofrefraction for abnormal light.

Namely, of the unpolarized light incident on such a cholestericstructure, either a right- or left-handed circularly polarized lightcomponent in the selective reflection wave range with the centralwavelength λ0 and the width Δλ is reflected owing to the above-describedpolarized-light-separating property, and the other circularly polarizedlight component and light (unpolarized light) not in this selectivereflection wave range are transmitted. It is noted that the right- orleft-handed circularly polarized light component is reflected withoutundergoing reversion of the direction of rotation unlike in the case ofordinary reflection of light.

Since visible light that causes coloring is in a wave range between 380nm and 780 nm, it is preferable to make the cholesteric structure sothat the wavelength of light to be selectively reflected owing to thecholesteric structure is either 380 nm or less, or 780 or more. By somaking the cholesteric structure, it is possible to prevent suchproblems as coloring that occurs when incident light (visible light) isreflected, while allowing the C plate type retardation layer 12 tofunction as a negative C plate. That the wavelength of light to beselectively reflected is shorter than that of incident light is morepreferable because, in this case, the rotatory polarization actiondecreases.

Next, a modification of the retardation optical element according tothis embodiment will be described with reference to FIG. 2.

As shown in FIG. 2, a retardation optical element 20 comprises: a Cplate type retardation layer 22 having a cholesteric structure withliquid crystalline molecules in planar orientation; and an A plate typeretardation layer 14 having a nematic structure, laminated adjacently tothe C plate type retardation layer 22.

Of these two retardation layers, the C plate type retardation layer 22functions as a negative C plate like the C plate type retardation layer12 in the retardation optical element 10 shown in FIG. 1, and has twomain opposite surface (surfaces with larger areas) 22A and 22B that areperpendicular to the direction of thickness (the direction of the normal15).

The directions of the directors Cb of liquid crystalline molecules onthe surface 22B, situated on the A plate type retardation layer 14 side,one of the two main opposite surfaces 22A and 22B of the C plate typeretardation layer 22, are substantially parallel to the directions ofthe directors Ca of liquid crystalline molecules on the other surface22A, situated on the side apart from the A plate type retardation layer14. It is preferable that the angle made by the direction (meandirection) of the directors Cb of liquid crystalline molecules on onesurface 22B of the C plate type retardation layer 22 and the direction(mean direction) of the directors Ca of liquid crystalline molecules onthe other surface 22A of the C plate type retardation layer 22 be within±10°, preferably within ±5°, more preferably within ±1°.

Further, the directions of the directors Ca of liquid crystallinemolecules on the surface 22A, situated on the side apart from the Aplate type retardation layer 14, of the C plate type retardation layer22 are substantially parallel to the directions of the directors Nb ofliquid crystalline molecules on the surface 14B, situated on the sideapart from the C plate type retardation layer 22, of the A plate typeretardation layer 14. It is preferable that the angle made by thedirection (mean direction) of the directors Ca of liquid crystallinemolecules on the surface 22A of the C plate type retardation layer 22and the direction (mean direction) of the directors Nb of liquidcrystalline molecules on the surface 14B of the A plate type retardationlayer 14 be within ±10°, preferably within ±5°, more preferably within±1°.

In the retardation optical element 20 shown in FIG. 2, in order to makethe directions of the directors Ca and Cb on the two main oppositesurfaces 22A and 22B of the C plate type retardation layer 22 be thesame (i.e., parallel to each other) with high accuracy, it is proper tomake the thickness of the C plate type retardation layer 22 equal to(0.5×integer) times the helical pitch p in the cholesteric structure(helical structure), thereby forming, between the surfaces 22A and 22B,a helical structure with a pitch number substantially equal to(0.5×integer). If so made, the thickness can optically be divided,without a remainder, by a half of the helical pitch p in the cholestericstructure consisting of liquid crystalline molecules, asdiagrammatically shown in FIGS. 3(A), 3(B) and 3(C), for example. Therecan thus be avoided optical deviation from the above equation (1) whichis a simplified theoretical equation, especially disturbance of thestate of polarization that is caused by phase shift that occurs for thelight entering along the helical axis.

The retardation optical element 20 shown in FIG. 2 is basically the sameas the retardation optical element 10 shown in FIG. 1 except thestructure of the C plate type retardation layer functioning as anegative C plate, so that the detailed explanations for the other pointsof the construction of the retardation optical element 20 are omitted.

It is possible to use, as materials for the C plate type retardationlayers 12 and 22 and the A plate type retardation layer 14 of theretardation optical elements 10 and 20, three-dimensionallycross-linkable liquid crystalline monomers and oligomers (polymerizablemonomers and oligomers), as well as polymeric liquid crystals (liquidcrystalline polymers) that can be solidified into its glassy state bycooling.

In the case where the C plate type retardation layers 12 and 22 and theA plate type retardation layer 14 are made from three-dimensionallycross-linkable, polymerizable monomers, it is possible to use mixturesof liquid crystalline monomers and chiral compounds such as aredisclosed in Japanese Laid-Open Patent Publication No. 258638/1995 andPublished Japanese Translation No. 508882/1998 of PCT InternationalPublication for Patent Application. If three-dimensionallycross-linkable, polymerizable oligomers are used, it is desirable to usecyclic organopolysiloxane compounds or the like having cholestericphases such as are disclosed in Japanese Laid-Open Patent PublicationNo. 165480/1982. By “three-dimensional cross-linking” is herein meantthat polymerizable monomer or oligomer molecules are three-dimensionallypolymerized to give a network structure. By making the molecules intosuch a state, it is possible to optically fix the liquid crystallinemolecules while maintaining their structure cholesteric or nematic, andis thus possible to obtain a film that is easy to handle as an opticalfilm and stable at normal temperatures.

The case where three-dimensionally cross-linkable, polymerizablemonomers are used is now taken as an example. If a liquid crystallinemonomer is made into a liquid crystal phase at a predeterminedtemperature, it becomes a nematic liquid crystal. If a chiral agent isadded to this liquid crystalline monomer, a chiral nematic liquidcrystal (cholesteric liquid crystal) can be obtained. More specifically,it is possible to use liquid crystalline monomers represented by thegeneral formulae (1) to (11), for example. In liquid crystallinemonomers represented by the general formula (11), X is preferably aninteger of 2 to 5.

It is preferable to use, as the chiral agent, those compoundsrepresented by the general formulae (12) to (14), for example. In chiralagents having the general formulae (12) and (13), X is preferably aninteger of 2 to 12. In chiral agents having the general formula (14), Xis preferably an integer of 2 to 5. In the general formula (12), R⁴ ishydrogen or methyl group.

On the other hand, in the case where the C plate type retardation layers12 and 22 and the A plate type retardation layer 14 are made from liquidcrystalline polymers, it is possible to use polymers containing mesogengroups, which make the polymers liquid crystalline, in their main orside chains, or in both their main and side chains; polymeric,cholesteric liquid crystals having cholesteryl groups in their sidechains; such liquid crystalline polymers as are disclosed in JapaneseLaid-Open Patent Publication No. 133810/1997; such liquid crystallinepolymers as are disclosed in Japanese Laid-Open Patent Publication No.293252/1999; and so forth.

Next, a process of producing the retardation optical element 10 (20) ofthe above-described construction according to this embodiment will bedescribed below.

First Production Process

Firstly, a production process that is employed in the case wherepolymerizable monomers or oligomers are used as materials for the Cplate type retardation layer 12 (22) and for the A plate typeretardation layer 14 will be described with reference to FIGS. 4(A) to4(E).

(1) Formation of C Plate Type Retardation Layer

In this case, an alignment layer 17 is formed in advance on a glasssubstrate or a polymeric film such as a TAC (cellulose triacetate) film16, as shown in FIG. 4(A). A polymerizable monomer (or polymerizableoligomer) 18 that is cholesteric is then applied to this alignment layer17, as shown in FIG. 4(B), and is aligned by the alignment regulationpower of the alignment layer 17. At this time, the polymerizable monomer(or polymerizable oligomer) 18 applied forms a liquid crystal layer.

In the case where the polymerizable monomer (or polymerizable oligomer)18 is made into a liquid crystal layer at a predetermined temperature,this liquid crystal layer becomes nematic. If any chiral agent is addedto this nematic liquid crystal, a chiral nematic liquid crystal(cholesteric liquid crystal) is obtained. Specifically, it is proper toadd a chiral agent to the polymerizable monomer or oligomer in an amountof approximately several percents to 10%, for example. By varying thechiral power by changing the type of the chiral agent to be added, or byvarying the concentration of the chiral agent in the polymerizablemonomer or oligomer, it is possible to control the wavelength of lightto be selectively reflected owing to the molecular structure of thepolymerizable monomer or oligomer. The polymerizable monomer (orpolymerizable oligomer) 18 is herein so prepared that the resultingsolid polymer selectively reflects light whose wavelength falls in awave range that is different from the wave range of incident light.

In order to decrease the viscosity of the polymerizable monomer (orpolymerizable oligomer) 18 for easy application, it may be dissolved, asneeded, in a solvent such as toluene or MEK to give a coating liquid. Inthis case, it is necessary to effect the drying step of evaporating thesolvent before the step of three-dimensionally cross-linking thepolymerizable monomer (or polymerizable oligomer) 18 by the applicationof ultraviolet light or an electron beam. Preferably, after effectingthe step of applying the coating liquid, the drying step is effected toevaporate the solvent; subsequently, the layer applied is held at atemperature at which it becomes a liquid crystal layer, and the step ofaligning the liquid crystal is then effected.

Next, with this state of alignment retained, that is, with thedirections of the directors of liquid crystalline molecules on thesurface, on the alignment layer 17 side, of the polymerizable monomer(or polymerizable oligomer) 18 being regulated by the alignmentregulation power of the surface of the alignment layer 17, thepolymerization of the polymerizable monomer (or polymerizable oligomer)18 is, as shown in FIG. 4(C), initiated by the combination use of aphotopolymerization initiator previously added and ultraviolet lightexternally applied, or initiated directly by the application of anelectron beam, thereby three-dimensionally cross-linking (polymerizing)the polymerizable monomer (or polymerizable oligomer) 18 forsolidification. Thus, there is formed a C plate type retardation layer12 that functions as a negative C plate such as is described above.

In the above process, if the alignment layer 17 is so formed that itsentire surface exerts alignment regulation power in substantially onedirection, the directions of the directors of liquid crystallinemolecules on the surface 12A, in contact with the alignment layer 17, ofthe C plate type retardation layer 12 can be made substantially the sameover the contact face.

In this case, in order to make the directions of the directors of liquidcrystalline molecules on the surface 12B, situated on the side apartfrom the alignment layer 17, of the C plate type retardation layer 12 besubstantially the same over the entire area of the surface 12B, it isproper to make the thickness of the C plate type retardation layer 12uniform. Further, as shown in FIGS. 5(A) to 5(D), a second alignmentlayer 17A may be laid on the polymerizable monomer (polymerizableoligomer) 18 (FIG. 5(C)) after applying the polymerizable monomer(polymerizable oligomer) 18 to the alignment layer 17 and beforethree-dimensionally cross-linking the polymerizable monomer(polymerizable oligomer) 18 in a series of the steps shown in FIGS. 4(A)to 4(C). By doing so, it is possible to make, with higher certainty, thedirections of the directors of liquid crystalline molecules on thesurface 12B of the C plate type retardation layer 12 be substantiallythe same over the entire area of the surface 12B.

In this condition, the polymerizable monomer (polymerizable oligomer) 18present between the alignment layer 17 and the second alignment layer17A is three-dimensionally cross-linked by the application ofultraviolet light or an electron beam, as in the step shown in FIG.4(C). Thus, there is formed a C plate type retardation layer 12 thatfunctions as a negative C plate such as is mentioned previously (FIG.5(D)).

It is proper to separate the second alignment layer 17A from the C platetype retardation layer 12 after the step of applying ultraviolet lightor an electron beam.

It is possible to form the alignment layer 17 and/or second alignmentlayer 17A by a conventionally known method. For example, the alignmentlayer may be formed by a method in which a PI (polyimide) or PVA(polyvinyl alcohol) film is formed on a glass substrate or a polymericfilm such as a TAC film 16 such as is described above and is thenrubbed, or a method in which a polymeric compound film capable ofserving as an optical alignment layer is formed on a glass substrate ora polymeric film such as a TAC film 16 and is irradiated with polarizedUV (ultraviolet light). Moreover, oriented PET (polyethyleneterephthalate) films, etc. may also be used for the alignment layer 17and/or second alignment layer 17A.

In the case where a polymeric film such as a TAC film (organic material)is used as the substrate on which the alignment layer 17 is formed, itis preferable to previously provide a barrier layer having resistance tosolvents, such as a PVA (polyvinyl alcohol) layer, on the polymericfilm, so that the substrate is not damaged by a solvent in which thepolymerizable monomer (or polymerizable oligomer) 18 is dissolved togive a coating liquid; the coating liquid is then applied to thisbarrier layer. In the case where a PVA layer is used as the barrierlayer, if this PVA layer is rubbed, the barrier layer is to serve alsoas an alignment layer.

(2) Formation of a Plate Type Retardation Layer

Thereafter, as shown in FIG. 4(D), separately-prepared anotherpolymerizable monomer (or polymerizable oligomer) 19 that is nematic anddevelops a nematic liquid crystal phase at a predetermined temperatureis applied directly to the C plate type retardation layer 12 that hasbeen formed in the above-described manner, and is aligned by thealignment regulation power of the surface 12B of the C plate typeretardation layer 12. At this time, the polymerizable monomer (orpolymerizable oligomer) 19 applied forms a liquid crystal layer.

In order to decrease the viscosity of the polymerizable monomer (orpolymerizable oligomer) 19 for easy application, it may be dissolved, asneeded, in a solvent such as toluene or MEK to give a coating liquid, asin the case of the polymerizable monomer (or polymerizable oligomer) 18.In this case, it is necessary to effect the drying step of evaporatingthe solvent before the step of three-dimensionally cross-linking thepolymerizable monomer (or polymerizable oligomer) 19 by the applicationof ultraviolet light or an electron beam. Preferably, after effectingthe step of applying the coating liquid, the drying step is effected toevaporate the solvent; subsequently, the layer applied is held at atemperature at which it becomes a liquid crystal layer, and the step ofaligning the liquid crystal is then effected.

Next, with this state of alignment retained, that is, with thedirections of the directors of liquid crystalline molecules on thesurface, situated on the C plate type retardation layer 12 side, of thepolymerizable monomer (or polymerizable oligomer) 19 being regulated bythe alignment regulation power of the surface of the C plate typeretardation layer 12, the polymerization of the polymerizable monomer(or polymerizable oligomer) 19 is, as shown in FIG. 4(E), initiated bythe combination use of a photopolymerization initiator previously addedand ultraviolet light externally applied, or initiated directly by theapplication of an electron beam, thereby three-dimensionallycross-linking (polymerizing) the polymerizable monomer (or polymerizableoligomer) 19 for solidification. Thus, there is formed an A plate typeretardation layer 14 that functions as an A plate such as is describedabove.

In order to make the directions of the directors of liquid crystallinemolecules on the surface 14B, situated on the side apart from the Cplate type retardation layer 12, of the A plate type retardation layer14 be substantially the same over the entire area of the surface 14B, itis proper to make the thickness of the C plate type retardation layer 12uniform, and, at the same time, the thickness of the A plate typeretardation layer 14 uniform. Alternatively, when conductingthree-dimensional cross-linking for solidifying the C plate typeretardation layer 12, such a second alignment layer 17A as is shown inFIGS. 5(A) to 5(D) may be used, and, in addition, when conductingthree-dimensional cross-linking for solidifying the A plate typeretardation layer 14, a second alignment layer that is the same as thesecond alignment layer 17A shown in FIGS. 5(C) and 5(D) may be providedon the surface, situated on the side apart from the surface 12B of the Cplate type retardation layer 12, of the polymerizable monomer(polymerizable oligomer) 19.

In producing the retardation optical element 20 shown in FIG. 2, it isnecessary to make the directions of the directors Cb of liquidcrystalline molecules on the surface 22B, situated on the side apartfrom the alignment layer 17, of the C plate type retardation layer 22 bethe same as the directions of the directors Ca of liquid crystallinemolecules on the surface 22A, situated on the alignment layer 17 side,of the C plate type retardation layer 22, and to make the directions ofthe directors Nb of liquid crystalline molecules on the surface 14B,situated on the side apart from the C plate type retardation layer 22,of the A plate type retardation layer 14 be the same as the directionsof the directors Ca of liquid crystalline molecules on the surface 22A,situated on the side apart from the A plate type retardation layer 14,of the C plate type retardation layer 22. To fulfill the aboverequirements, it is proper to adjust the coating thickness of a liquidcrystal so that the thickness of the C plate type retardation layer 22and that of the A plate type retardation layer 14 are equal to(0.5×integer) times the helical pitch in the helical structureconsisting of liquid crystalline molecules. Alternatively, such a secondalignment layer 17A as is shown in FIGS. 5(C) and 5(D) may be used. Ifused, the second alignment layer 17A is brought into contact with thesurface 22B, situated on the side opposite to the alignment layer 17, ofthe C plate type retardation layer 22, or with the surface 14B, situatedon the side opposite to the C plate type retardation layer 22, of the Aplate type retardation layer 14.

Thus, there is produced a retardation optical element 10 (20) in whichthe C plate type retardation layer 12 (22) and the A plate typeretardation layer 14 are laminated adjacently to each other.

Second Production Process

Next, a production process that is employed in the case where liquidcrystalline polymers are used as materials for the C plate typeretardation layer 12 (22) and for the A plate type retardation layer 14will be described with reference to FIGS. 6(A) to 6(E).

(1) Formation of C Plate Type Retardation Layer

In this case, an alignment layer 17 is formed in advance on a glasssubstrate or a polymeric film such as a TAC film 16, as shown in FIG.6(A). A liquid crystalline polymer 32 that is cholesteric is applied tothis alignment layer 17, as shown in FIG. 6(B), and is aligned by thealignment regulation power of the alignment layer 17. At this time, theliquid crystalline polymer 32 applied forms a liquid crystal layer.

A cholesteric liquid crystalline polymer in which a liquid crystallinepolymer itself has chiral power may be used as it is as the liquidcrystalline polymer 32. Alternatively, a mixture of a nematic liquidcrystalline polymer and a cholesteric liquid crystalline polymer may beused as the liquid crystalline polymer 32. Specifically, for example, itis possible to use polymers containing mesogen groups, which make thepolymers liquid crystalline, in their main or side chains, or in boththeir main and side chains; polymeric, cholesteric liquid crystalshaving cholesteryl groups in their side chains; such liquid crystallinepolymers as are disclosed in Japanese Laid-Open Patent Publication No.133810/1997; such liquid crystalline polymers as are disclosed inJapanese Laid-Open Patent Publication No. 293252/1999; and so forth.

The state of such a liquid crystalline polymer 32 changes withtemperature. For example, a liquid crystalline polymer 32 having a glasstransition temperature of 90° C. and an isotropic transition temperatureof 200° C. is in the cholesteric liquid crystalline state attemperatures between 90° C. and 200° C.; by cooling to room temperature,it is possible to solidify this polymer into its the glassy state whilemaintaining its structure cholesteric.

To adjust the wavelength of incident light that is selectively reflectedowing to the cholesteric structure of the liquid crystalline polymer 32,it is proper to control the chiral power in the liquid crystallinemolecules by a known method if a cholesteric liquid crystalline polymeris used as the liquid crystalline polymer 32. In the case where amixture of a nematic liquid crystalline polymer and a cholesteric liquidcrystalline polymer is used, it is, for this purpose, proper to adjustthe mixing ratio of these two components.

In order to decrease the viscosity of the liquid crystalline polymer 32for easy application, it may be dissolved, as needed, in a solvent suchas toluene or MEK to give a coating liquid. In this case, it isnecessary to effect the drying step of evaporating the solvent beforethe step of cooling the liquid crystalline polymer 32. Preferably, aftereffecting the step of applying the coating liquid, the drying step iseffected to evaporate the solvent, and the step of aligning the liquidcrystal layer is then effected.

With this state of alignment retained, that is, with the directions ofthe directors of liquid crystalline molecules on the surface, situatedon the alignment layer 17 side, of the liquid crystalline polymer 32being regulated, the liquid crystalline polymer 32 is cooled to atemperature below its glass transition temperature (Tg), as shown inFIG. 6(C), thereby solidifying the polymer into its glassy state. Thus,there is formed a C plate type retardation layer 12 that functions as anegative C plate such as is described above.

In the above process, if the alignment layer 17 is so formed that itsentire surface exerts alignment regulation power in substantially onedirection, it is possible to make the directions of the directors ofliquid crystalline molecules on the surface 12A, in contact with thealignment layer 17, of the C plate type retardation layer 12 besubstantially the same over the contact face.

In this case, in order to make the directions of the directors of liquidcrystalline molecules on the surface 12B, situated on the side apartfrom the alignment layer 17, of the C plate type retardation layer 12 besubstantially the same over the entire area of the surface 12B, it isproper to make the thickness of the C plate type retardation layer 12uniform. Alternatively, such a second alignment layer 17A as is shown inFIGS. 5(C) and 5(D) may also be provided on the surface, situated on theside apart from the alignment layer 17, of the liquid crystallinepolymer 32. By doing so, it is possible to make, with higher certainty,the directions of the directors of liquid crystalline molecules on thesurface 12B of the C plate type retardation layer 12 be substantiallythe same over the entire area of the surface 12B. It is proper toseparate the second alignment layer 17A from the C plate typeretardation layer 12 after the cooling step.

In this production process, the alignment layer 17 and/or secondalignment layer 17A may be the same as those in the aforementioned firstproduction process. In the case where a polymeric film such as a TACfilm (organic material) is used as the substrate on which the alignmentlayer 17 is formed, it is preferable to previously provide, as in theabove-described first production process, a barrier layer havingresistance to solvents, such as a PVA (polyvinyl alcohol) layer, on thepolymeric film, so that the substrate is not damaged by a solvent inwhich the liquid crystalline polymer 32 is dissolved to give a coatingliquid; the coating liquid is then applied to this barrier layer.

(2) Formation of a Plate Type Retardation Layer

Thereafter, as shown in FIG. 6(D), separately-prepared another liquidcrystalline polymer 34 that is nematic and develops a nematic liquidcrystal phase at a predetermined temperature is applied directly to theC plate type retardation layer 12 that has been formed in theabove-described manner, and is aligned by the alignment regulation powerof the surface 12B of the C plate type retardation layer 12. At thistime, the liquid crystalline polymer 34 applied forms a liquid crystallayer.

Such a nematic liquid crystalline polymer as is described in JapaneseLaid-Open Patent Publication No. 293252/1999 mentioned previously, forexample, is used as the liquid crystalline polymer 34.

The state of such a liquid crystalline polymer 34 changes withtemperature. The liquid crystalline polymer 34 is in the nematic liquidcrystalline state at temperatures in a predetermined temperature range;by cooling to room temperature, it is possible to solidify this liquidcrystalline polymer 34 into its glassy state while maintaining itsstructure nematic.

In order to decrease the viscosity of the liquid crystalline polymer 34for easy application, it may be dissolved, as needed, in a solvent suchas toluene or MEK to give a coating liquid as in the case of the liquidcrystalline polymer 32. In this case, it is necessary to effect thedrying step of evaporating the solvent before the step of cooling theliquid crystalline polymer 34. Preferably, after effecting the step ofapplying the coating liquid, the drying step is effected to evaporatethe solvent, followed by the step of aligning the liquid crystal.

Next, with this state of alignment retained, that is, with thedirections of the directors of liquid crystalline molecules on thesurface, situated on the C plate type retardation layer 12 side, of theliquid crystalline polymer 34 being regulated by the alignmentregulation power of the surface of the C plate type retardation layer12, the liquid crystalline polymer 34 is cooled to a temperature belowits glass transition temperature (Tg), as shown in FIG. 6(E), therebysolidifying the polymer into its glassy state. Thus, there is formed anA plate type retardation layer 14 that functions as an A plate such asis described above.

In order to make the directions of the directors of liquid crystallinemolecules on the surface 14B, situated on the side apart from the Cplate type retardation layer 12, of the A plate type retardation layer14 be substantially the same over the entire area of the surface 14B, itis proper to make the C plate type retardation layer 12 to have auniform thickness, and, at the same time, the A plate type retardationlayer 14 to have a uniform thickness. Alternatively, when solidifyingthe C plate type retardation layer 12 by cooling, such a secondalignment layer 17A as is shown in FIGS. 5(A) to 5(D) may be used, and,in addition, when solidifying the A plate type retardation layer 14 bycooling, a second alignment layer that is the same as the secondalignment layer 17A shown in FIGS. 5(C) and 5(D) may be provided on thesurface, situated on the side apart from the surface 12B of the C platetype retardation layer 12, of the liquid crystalline polymer 34.

In producing the retardation optical element 20 shown in FIG. 2, it isnecessary to make the directions of the directors Cb of liquidcrystalline molecules on the surface 22B, on the side apart from thealignment layer 17, of the C plate type retardation layer 22 be the sameas the directions of the directors Ca of liquid crystalline molecules onthe surface 22A, situated on the alignment layer 17 side, of the C platetype retardation layer 22, and to make the directions of the directorsNb of liquid crystalline molecules on the surface 14B, situated on theside apart from the C plate type retardation layer 22, of the A platetype retardation layer 14 be the same as the directions of the directorsCa of liquid crystalline molecules on the surface 22A, situated on theside apart from the A plate type retardation layer 14, of the C platetype retardation layer 22. To fulfill the above requirements, thecoating thickness of a liquid crystal may be adjusted so that thethickness of the C plate type retardation layer 22 and that of the Aplate type retardation layer 14 are equal to (0.5×integer) times thehelical pitch in the helical structure consisting of liquid crystallinemolecules. Alternatively, such a second alignment layer 17A as is shownin FIGS. 5(C) and 5(D) may be used. When used, the second alignmentlayer 17A is brought into contact with the surface 22B, situated on theside opposite to the alignment layer 17, of the C plate type retardationlayer 22, or with the surface 14B, situated on the side opposite to theC plate type retardation layer 22, of the A plate type retardation layer14.

Thus, there is produced a retardation optical element 10 (20) in whichthe C plate type retardation layer 12 (22) and the A plate typeretardation layer 14 are laminated adjacently to each other.

In all of the above-described embodiments, the C plate type retardationlayer 12 (22) having a cholesteric structure is firstly formed on thealignment layer 17 that has been formed on a glass substrate or apolymeric film such as a TAC film 16, and the A plate type retardationlayer 14 having a nematic structure is then formed on this C plate typeretardation layer 12 (22). The present invention is not limited to this,and the retardation optical element may also be produced by firstlyforming the A plate type retardation layer 14 having a nematicstructure, and then forming, on this A plate type retardation layer 14,the C plate type retardation layer 12 (22) having a cholestericstructure. In this case, a cholesteric liquid crystal is applieddirectly to the A plate type retardation layer 14 and is then solidifiedwith the directions of the directors of liquid crystalline molecules onthe surface, situated on the A plate type retardation layer 14 side, ofthe liquid crystal being regulated by the alignment regulation power ofthe surface of the A plate type retardation layer 14, thereby formingthe C plate type retardation layer 12 (22). The other procedures,conditions, and so forth in this production process are basically thesame as those in the above-described production process, so thatdetailed explanations for them are omitted.

Further, in all of the above-described embodiments, the retardationoptical element has a two-layered structure composed of the single, Cplate type retardation layer 12 (22) and the single, A plate typeretardation layer 14. The present invention is not limited to this, andthe retardation optical element can have a structure consisting of threeor more layers in which at least one of the above-described C plate typeretardation layer and A plate type retardation layer is made from two ormore layers. If so made, the retardation optical element can providemore diversified types of optical compensation.

The retardation optical elements 10 and 20 according to theaforementioned embodiments can be incorporated into such a liquidcrystal display 60 as is shown in FIG. 7, for example.

The liquid crystal display 60 shown in FIG. 7 comprises a polarizer 102Aon the incident side, a polarizer 102B on the emergent side, and aliquid crystal cell 104.

Of these components, the polarizers 102A and 102B are so made that theyselectively transmit only linearly polarized light having the plane ofvibration in a predetermined direction, and are arranged in the crossnicol disposition so that the direction of vibration of the linearlypolarized light transmitted by the polarizer 102A is perpendicular tothat of vibration of the linearly polarized light transmitted by thepolarizer 102B. The liquid crystal cell 104 contains a large number ofcells corresponding to pixels and is placed between the polarizers 102Aand 102B.

In the liquid crystal display 60, the liquid crystal cell 104 is of VAmode, in which a nematic liquid crystal having negative dielectricanisotropy is sealed in a liquid crystal cell. Linearly polarized lightthat has passed through the polarizer 102A on the incident side passes,without undergoing phase shift, through those cells in the liquidcrystal cell 104 that are in the non-driven state, and is blocked by thepolarizer 102 on the emergent side. On the contrary, when the linearlypolarized light passes through those cells in the liquid crystal cell104 that are in the driven state, it undergoes phase shift, and thisphase-shifted light passes through and emerges from the polarizer 102Bon the emergent side in an amount corresponding to the amount of thephase shift. It is therefore possible to display the desired image onthe emergent-side polarizer 102B side by properly, individuallycontrolling the driving voltage that is applied to each cell in theliquid crystal cell 104.

In the liquid crystal display 60 of the above-described construction,the retardation optical element 10 (20) according to the aforementionedembodiment is placed between the liquid crystal cell 104 and thepolarizer 102B on the emergent side (a polarizer capable of selectivelytransmitting light in a predetermined state of polarization, emergingfrom the liquid crystal cell 104). Of the light in a predetermined stateof polarization, emerging from the liquid crystal cell 104, the lightemerging slantingly in the direction deviating from the normal to theliquid crystal cell 104 can be optically compensated by the retardationoptical element 10 (20) for the state of polarization.

It is herein preferable to place the retardation optical element 10 (20)so that the C plate type retardation layer 12 (22) faces to the liquidcrystal cell 104 and that the A plate type retardation layer 14 faces tothe polarizer 102B. By so placing the retardation optical element 10(20), it is possible to effectively obtain the desired performances.

As mentioned above, according to the liquid crystal display 60 of theabove-described construction, the retardation optical element 10 (20)according to the aforementioned embodiment is placed between the liquidcrystal cell 104 and the polarizer 102B on the emergent side, whereby,of the light emerging from the liquid crystal cell 104, the lightemerging slantingly in the direction deviating from the normal to theliquid crystal cell 104 is optically compensated by the retardationoptical element for the state of polarization. The retardation opticalelement can, therefore, prevent the appearance of bright-and-darkpatterns on the liquid crystal display 60 and, moreover, improvecontrast, while effectively improving viewing angle dependency; loweringof display quality can thus be prevented.

The liquid crystal display 60 shown in FIG. 7 is of transmission type,which light is transmitted from one side to the other in the directionof thickness. This embodiment is not limited to this type of display,and the retardation optical element 10 (20) according to theabove-described embodiment may be incorporated also into a liquidcrystal display of reflection type or of reflection/transmission type.

Further, in the liquid crystal display 60 shown in FIG. 7, theretardation optical element 10 (20) according to the above-describedembodiment is placed between the liquid crystal cell 104 and thepolarizer 102B on the emergent side. However, depending on the type ofoptical compensation desired, the retardation optical element 10 (20)may be placed between the liquid crystal cell 104 and the polarizer 102Aon the incident side. Furthermore, the retardation optical element 10(20) may be placed on both sides of the liquid crystal cell 104 (betweenthe liquid crystal cell 104 and the polarizer 102A on the incident side,and between the liquid crystal cell 104 and the polarizer 102B on theemergent side). It is noted that not only one but also a plurality ofretardation optical elements may be placed between the liquid crystalcell 104 and the polarizer 102A on the incident side, or between theliquid crystal cell 104 and the polarizer 102B on the emergent side. Inall of these cases, it is preferable to place the retardation opticalelement 10 (20) so that the C plate type retardation layer 12 faces tothe liquid crystal cell 104 and that the A plate type retardation layer14 faces to the polarizer 102A or 102B.

EXAMPLES

Examples of the aforementioned embodiments will now be given togetherwith Comparative Examples.

Example 1

In Example 1, a retardation layer functioning as a negative C plate anda retardation layer functioning as an A plate were made uniform inthickness, whereby the directions of the directors of liquid crystallinemolecules on the surfaces of each retardation layer were made the same.

A solution (chiral nematic liquid crystal solution) was prepared bydissolving, in toluene, 90 parts of a monomer containing, in itsmolecule, polymerizable acrylates at both ends and spacers betweenmesogen existing at the center and the acrylates, having anematic-isotropic transition temperature of 110° C. (i.e., a monomerhaving a molecular structure represented by the above chemical formula(11)) and 10 parts of a chiral agent having, in its molecule,polymerizable acrylates at both ends (i.e., a compound having amolecular structure represented by the above chemical formula (14)). Tothis toluene solution, a photopolymerization initiator (“Irgacure® 907”available from Chiba Specialty Chemicals K.K., Japan) was added in anamount of 5% by weight of the above-described monomer. (With respect tothe chiral nematic liquid crystal thus obtained, it was confirmed thatthe directors of liquid crystalline molecules were oriented at thesurface of the alignment layer in the direction within the direction ofrubbing ±5 degrees.)

On the other hand, a transparent glass substrate was spin-coated withpolyimide (“Optomer® AL1254” manufactured by JSR Corporation, Japan)dissolved in a solvent. After drying, a film of the polyimide was formedat 200° C. (film thickness 0.1 μm) and was rubbed in one direction inorder to make the film function as an alignment layer.

The glass substrate coated with the alignment layer was set in aspin-coater and was spin-coated with the toluene solution of theabove-described monomer, etc. under such conditions that the resultingfilm had a thickness as uniform as possible.

The toluene contained in the above toluene solution was then evaporatedat 80° C. to form a coating film. A glass substrate with an alignmentlayer (second alignment layer) prepared separately was placed on thesurface of the coating film on the side opposite to the above-describedglass substrate with respect to the alignment layer (first alignmentlayer), thereby sandwiching the coating film. In this process, the firstand second alignment layers were made the same in terms of the directionof rubbing.

Ultraviolet light was applied to the above coating film, and withradicals thus released from the photopolymerization initiator containedin the coating film, the acrylates in the monomer molecules werethree-dimensionally cross-linked for solidification (polymerization),thereby forming a layer having a cholesteric structure. At this time,the separately prepared glass substrate with the alignment layer (secondalignment layer) described above was separated from the coating film.The thickness of the coating film was 2.0 μm±1.5% at this point. By themeasurement made by using a spectrophotometer, it was found that thecentral wavelength of the selective reflection wave range of the coatingfilm was 280 nm.

The above layer having a cholesteric structure formed in this manner wassubjected to measurements using an automatic birefringence measuringinstrument (trade name “KOBRA® 21ADH” manufactured by Oji Keisoku KikiKabushiki Kaisha, Japan). As a result, it was confirmed that this layerwas functioning as a negative C plate (retardation layer).

Next, the above layer having a cholesteric structure was spin-coatedwith a toluene solution (nematic liquid crystal solution) containing thesame components as those in the above-described toluene solution,provided that the chiral agent was not contained, under such conditionsthat the resulting film had a thickness as uniform as possible.

The toluene contained in the above toluene solution was then evaporatedat 80° C. to form a coating film. Ultraviolet light was applied to thiscoating film, and with radicals thus released from thephotopolymerization initiator contained in the coating film, theacrylates in the monomer molecules were three-dimensionally cross-linkedfor solidification (polymerization), thereby forming a layer having anematic structure.

Thus, there was finally produced a retardation optical element in whichthe layer having a cholesteric structure and the layer having a nematicstructure were laminated adjacently to each other. The total thicknessof this retardation optical element was 3.5 μm±1.5%.

The retardation optical element produced in this manner was subjected tomeasurements using an automatic birefringence measuring instrument(trade name “KOBRA® 21ADH” manufactured by Oji Keisoku Kiki KabushikiKaisha, Japan). As a result, it was confirmed that this retardationoptical element was functioning as both a negative C plate and an Aplate.

Further, the cross section of the layer having a cholesteric structure(the retardation layer functioning as a negative C plate) and that ofthe layer having a nematic structure (the retardation layer functioningas an A plate) were observed using a transmission electron microscope.As a result, the bright-and-dark patterns that appeared in theretardation layer functioning as a negative C plate were found to beparallel to each other (from this, it is understood that the helicalaxes in the retardation layer functioning as a negative C plate extendin the same direction). On the other hand, no bright-and-dark patternsappeared in the retardation layer functioning as an A plate (from this,it is understood that the directions of the directors of liquidcrystalline molecules in the retardation layer functioning as an A plateare the same). Moreover, the contrasts on the two main opposite surfacesof the retardation layer functioning as an A plate were the same, andthe contrasts on the two main opposite surfaces of the retardation layerfunctioning as a negative C plate were also the same (from this, it isunderstood that the directions of the directors of liquid crystallinemolecules on the two main opposite surfaces of the retardation layerfunctioning as an A plate are the same, and that the directions of thedirectors of liquid crystalline molecules on the two main oppositesurfaces of the retardation layer functioning as a negative C plate arealso the same).

Furthermore, the retardation optical element 10 produced was placedbetween linear polarizers 70A and 70B arranged in the cross nicoldisposition, as shown in FIG. 8, and was visually observed. As a result,the bright-and-dark patterns observed on the plane were very few.

Comparative Example 1

In Comparative Example 1, the procedure of Example 1 was repeated,provided that the thickness of the retardation layer functioning as anegative C plate was made non-uniform in order to make the directions ofthe directors of liquid crystalline molecules be different from oneanother.

Namely, a retardation optical element was produced in the same manner asin Example 1, except that the thickness of the layer having acholesteric structure (the retardation layer functioning as a negative Cplate) was made 2.0 μm±5% by changing the settings for the spin coaterand that the second alignment layer was not used. This retardationoptical element was observed in the same way as in Example 1. As aresult, bright-and-dark patterns were clearly observed on the plane.

Comparative Example 2

In Comparative Example 2, the procedure of Example 1 was repeated,provided that the alignment layer on which the retardation layerfunctioning as a negative C plate would be formed was obtained byrubbing the coating film in various directions in order to make thedirections of the directors of liquid crystalline molecules be differentfrom one another.

Namely, a retardation optical element was produced in the same manner asin Example 1, except that the alignment layer was obtained by rubbingthe coating film in different directions. This retardation opticalelement was observed in the same way as in Example 1. As a result,bright-and-dark patterns were clearly observed on the plane.

Example 2

In Example 2, a retardation layer functioning as a negative C plate wasmade uniform in thickness, and the helical pitches were made equal,whereby the directions of the directors of liquid crystalline moleculeson the two main opposite surfaces of this layer were made parallel toeach other.

Namely, a retardation optical element was produced in the same manner asin Example 1, except that considering the refractive index of thematerial to be used, the retardation layer functioning as a negative Cplate was made to have such a thickness that the directions of thedirectors at the starting point and end of the cholesteric structurewere parallel to each other. This retardation optical element wasobserved in the same way as in Example 1. As a result, thebright-and-dark patterns that appeared on the plane were found obviouslyfewer than those patterns that appeared when the retardation opticalelement containing the retardation layer whose thickness was differentfrom the above-described thickness was observed.

The retardation optical element 20 produced was, as shown in FIG. 8,placed between linear polarizers 70A and 70B arranged in the cross nicoldisposition and was visually observed. As a result, the bright-and-darkpatterns observed on the plane were extremely few. Further, the linearpolarizers 70A and 70B (see FIG. 8) placed on both sides of theretardation optical element 20 produced were individually rotated, andthe angle made by the direction of the director at the starting point ofthe cholesteric structure and that of the director at the end of thisstructure was visually determined by the intensity of transmitted light.As a result, this angle was found to be within ±5 degrees.

Comparative Example 3

In Comparative Example 3, the procedure of Example 2 was repeated,provided that the thickness of the retardation layer functioning as anegative C plate was made non-uniform in order to make the directions ofthe directors of liquid crystalline molecules different from oneanother.

Namely, a retardation optical element was produced in the same manner asin Example 2, except that the thickness of the layer having acholesteric structure (the retardation layer functioning as a negative Cplate) was made 2.0 μm±5% by changing the settings for the spin coaterand that the second alignment layer was not used. This retardationoptical element was observed in the same way as in Example 2. As aresult, bright-and-dark patterns were clearly observed on the plane.

Example 3

In Example 3, liquid crystalline polymers were used as materials for aretardation layer functioning as a negative C plate and for aretardation layer functioning as an A plate, and these retardationlayers were made uniform in thickness, whereby the directions of thedirectors of liquid crystalline molecules on the surfaces of eachretardation layer were made the same.

A solution (polymeric, cholesteric liquid crystal solution) was preparedby dissolving, in toluene, a side-chain acrylic liquid crystallinepolymer having a glass transition temperature of 80° C. and an isotropictransition temperature of 200° C. (With respect to the polymeric,cholesteric liquid crystal thus obtained, it was confirmed that thedirectors of liquid crystalline molecules were oriented at the surfaceof the alignment layer in the direction within the direction of rubbing±5 degrees.)

On the other hand, a transparent glass substrate was spin-coated withpolyimide (“Optomer® AL1254” manufactured by JSR Corporation, Japan)dissolved in a solvent. After drying, a film of the polyimide was formedat 200° C. (film thickness 0.1 μm) and was rubbed in one direction inorder to make this film function as an alignment layer.

The glass substrate coated with the alignment layer was set in aspin-coater and was spin-coated with the toluene solution of theabove-described liquid crystalline polymer under such conditions thatthe resulting film had a thickness as uniform as possible.

The toluene contained in the above toluene solution was then evaporatedat 90° C. to form a coating film. A glass substrate with an alignmentlayer (second alignment layer) prepared separately was placed on thesurface of the coating film on the side opposite to the above-describedglass substrate with respect to alignment layer (first alignment layer),thereby sandwiching the coating film. In this process, the first andsecond alignment layers were made the same in terms of the direction ofrubbing.

The above coating film was held at 150° C. for 10 minutes, and it wasvisually confirmed by means of selective reflection that the coatingfilm was cholesteric. This coating film was then cooled to roomtemperature for solidifying the liquid crystalline polymer into itsglassy state, thereby forming a layer having a cholesteric structure. Atthis time, the separately prepared glass substrate with the alignmentlayer (second alignment layer) described above was separated from thecoating film. The thickness of the coating film was 2.0 μm±1.5% at thispoint. By the measurement made by using a spectrophotometer, it wasfound that the central wavelength of the selective reflection wave rangeof the coating film was 280 nm.

The above layer having a cholesteric structure formed in this manner wassubjected to measurements using an automatic birefringence measuringinstrument (trade name “KOBRA® 21ADH” manufactured by Oji Keisoku KikiKabushiki Kaisha, Japan). As a result, it was confirmed that this layerwas functioning as a negative C plate (retardation layer).

Next, the above layer having a cholesteric structure was spin-coatedwith a toluene solution containing a nematic liquid crystalline polymer(polymeric, nematic liquid crystal solution) under such conditions thatthe resulting film had a thickness as uniform as possible.

The toluene contained in the above toluene solution was then evaporatedat 90° C. to form a coating film. This coating film was cooled to roomtemperature for solidifying the liquid crystalline polymer into itsglassy state, thereby forming a layer having a nematic structure.

Thus, there was finally produced a retardation optical element in whichthe layer having a cholesteric structure and the layer having a nematicstructure were laminated adjacently to each other. The total thicknessof this retardation optical element was 3.5 μm±1.5%.

The retardation optical element produced in this manner was subjected tomeasurements using an automatic birefringence measuring instrument(trade name “KOBRA® 21ADH” manufactured by Oji Keisoku Kiki KabushikiKaisha, Japan). As a result, it was confirmed that this retardationoptical element was functioning as both a negative C plate and an Aplate.

Further, the cross section of the layer having a cholesteric structure(the retardation layer functioning as a negative C plate) and that ofthe layer having a nematic structure (the retardation layer functioningas an A plate) were observed using a transmission electron microscope.As a result, the bright-and-dark patterns that appeared in theretardation layer functioning as a negative C plate were found to beparallel to each other (from this, it is understood that the helicalaxes in the retardation layer functioning as a negative C plate extendin the same direction). On the other hand, no bright-and-dark patternsappeared in the retardation layer functioning as an A plate (from this,it is understood that the directions of the directors of liquidcrystalline molecules in the retardation layer functioning as an A plateare the same). Moreover, the contrasts on the two main opposite surfacesof the retardation layer functioning as an A plate were the same, andthe contrasts on the two main opposite surfaces of the retardation layerfunctioning as a negative C plate were also the same (from this, it isunderstood that the directions of the directors of liquid crystallinemolecules on the two main opposite surfaces of the retardation layerfunctioning as an A plate are the same and that the directions of thedirectors of liquid crystalline molecules on the two main oppositesurfaces of the retardation layer functioning as a negative C plate arealso the same).

Furthermore, the retardation optical element 10 produced was placedbetween linear polarizers 70A and 70B arranged in the cross nicoldisposition, as shown in FIG. 8, and was visually observed. As a result,the bright-and-dark patterns observed on the plane were very few.

Comparative Example 4

In Comparative Example 4, the procedure of Example 3 was repeated,provided that the thickness of the retardation layer functioning as anegative C plate was made non-uniform in order to make the direction ofthe directors of liquid crystalline molecules be different from oneanother.

Namely, a retardation optical element was produced in the same manner asin Example 3, except that the thickness of the layer having acholesteric structure (the retardation layer functioning as a negative Cplate) was made 2.0 μm±5% by changing the settings for the spin coaterand that the second alignment layer was not used. This retardationoptical element was observed in the same way as in Example 3. As aresult, bright-and-dark patterns were clearly observed on the plane.

1. A retardation optical element, comprising: a transmission type Cplate type retardation layer; and a transmission type A plate typeretardation layer laminated together with the C plate type retardationlayer so that a first surface of the A plate type retardation layercontacts a first surface of the C plate type retardation layer; wherein:the C plate retardation layer has a cholesteric structure includingliquid crystalline molecules in planar orientation, a helical pitch ofthe liquid crystalline molecules being configured such that the C platetype retardation layer selectively reflects light having a wavelength ina wave range that is different from a wave range of incident light; theC plate type retardation layer functions as a negative C plate; the Aplate type retardation layer has a nematic structure; the A plate typeretardation layer functions as an A plate; directors of liquidcrystalline molecules on the first surface of the C plate typeretardation layer and directors of liquid crystalline molecules on thefirst surface of the A plate type retardation layer are arranged insubstantially the same direction.
 2. The retardation optical elementaccording to claim 1, wherein the directors of liquid crystallinemolecules on the first surface of the C plate type retardation layer anddirectors of liquid crystalline molecules on a second surface of the Cplate type retardation layer are substantially parallel.
 3. Theretardation optical element according to claim 1, wherein the directorsof liquid crystalline molecules on a second surface of the C plate typeretardation layer and directors of liquid crystalline molecules on asecond surface of the A plate type retardation layer are substantiallyparallel.
 4. The retardation optical element according to claim 1,wherein the helical pitch of the liquid crystalline molecules of the Cplate type retardation layer has a pitch number substantially equal to(0.5×integer) between the first surface and a second surface oppositefrom the first surface.
 5. The retardation optical element according toclaim 1, wherein the C plate type retardation layer comprises chiralnematic liquid crystal solidified by three-dimensional cross-linking. 6.The retardation optical element according to claim 1, wherein the Cplate type retardation layer comprises polymeric, cholesteric liquidcrystal solidified into a glassy state.
 7. The retardation opticalelement according to claim 1, wherein the A plate type retardation layercomprises nematic liquid crystal solidified by three-dimensionalcross-linking.
 8. The retardation optical element according to claim 1,wherein the A plate type retardation layer comprises polymeric, nematicliquid crystal solidified into a glassy state.
 9. A liquid crystaldisplay comprising: a liquid crystal cell; a first polarizer provided ona first side of the liquid crystal cell; a second polarizer provided ona second side of the liquid crystal cell; and the retardation opticalelement according to claim 1; wherein: the retardation optical elementis located between the liquid crystal cell and one of the polarizers;and the retardation optical element compensates for predetermined stateof polarization of light, incident on and/or emerging from the liquidcrystal cell, emerging slantingly in a direction deviating from normalto the liquid crystal cell.
 10. A process of producing a retardationoptical element, comprising: applying a first liquid crystal havingcholesteric regularity to an alignment surface of an alignment layerhaving alignment regulation power in substantially one direction overthe entire alignment surface; solidifying the first liquid crystal toform a transmission type C plate type retardation layer that functionsas a negative C plate, wherein, when the first liquid crystal issolidified, directions of directors of liquid crystalline molecules on afirst surface of the C plate type retardation layer are regulated by thealignment regulation power of the alignment layer, and the C plate typeretardation layer selectively reflects light having a wavelength in awave range different from a wave range of incident light; applying asecond liquid crystal having a nematic regularity directly to the Cplate type retardation layer; and solidifying the second liquid crystalto form a transmission type A plate type retardation layer thatfunctions as an A plate, wherein, when the second liquid crystal issolidified, directions of directors of liquid crystalline molecules on afirst surface of the A plate type retardation layer adjacent to the theC plate type retardation layer are regulated by an alignment regulationpower of a second surface of the C plate type retardation layer.
 11. Theprocess according to claim 10, wherein: the first liquid crystalcomprises at least one of polymerizable cholesteric monomers andoligomers, the first liquid crystal being solidified bythree-dimensional cross-linking; and the second liquid crystal comprisesat least one of polymerizable nematic monomers and oligomers, the secondliquid crystal being solidified by three-dimensional cross-linking. 12.The process according to claim 10, wherein: the first liquid crystalcomprises a cholesteric liquid crystalline polymer, the first liquidcrystal being solidified into a glassy state by cooling; and the secondliquid crystal comprises a nematic liquid crystalline polymer, thesecond liquid crystal being solidified into a glassy state by cooling.13. The process according to claim 10, wherein applying the first liquidcrystal comprises applying the first liquid crystal in a coatingthickness selected so directors of liquid crystalline molecules on thefirst surface of the C plate type retardation layer and directors ofliquid crystalline molecules on the second surface of the C plate typeretardation layer are substantially parallel.
 14. The process accordingto claim 10, further comprising bringing a second alignment layer intocontact with a surface of the applied first liquid crystal opposite froma surface in contact with the alignment layer before solidifying thefirst liquid crystal so that, when the first liquid crystal issolidified, directions of directors of liquid crystalline molecules onthe first and second surfaces of the C plate type retardation layer areregulated.
 15. The process according to claim 10, further comprisingbringing a second alignment layer into contact with a surface of theapplied second liquid crystal opposite from a surface in contact withthe C plate type retardation layer before solidifying the second liquidcrystal so that, when the second liquid crystal is solidified,directions of directors of liquid crystalline molecules on the firstsurface of the A plate type retardation layer and a second surface ofthe A plate type retardation layer are regulated.
 16. A process ofproducing a retardation optical element, comprising: applying a firstliquid crystal having nematic regularity to an alignment surface of analignment layer having alignment regulation power in substantially onedirection over the entire alignment surface; solidifying the firstliquid crystal to form a transmission type A plate type retardationlayer that functions as an A plate, wherein, when the first liquidcrystal is solidified, directions of directors of liquid crystallinemolecules on a first surface of the A plate type retardation layer areregulated by the alignment regulation power of the alignment layer;applying a second liquid crystal having a cholesteric regularitydirectly to the A plate type retardation layer; and solidifying thesecond liquid crystal to form a transmission type C plate typeretardation layer that functions as a negative C plate, wherein, whenthe second liquid crystal is solidified, directions of directors ofliquid crystalline molecules on a first surface of the C plate typeretardation layer adjacent to the A plate type retardation layer areregulated by an alignment regulation power of a second surface of the Aplate type retardation layer, and the C plate type retardation layerselectively reflects light having a wavelength in a wave range differentfrom a wave range of incident light.
 17. The process according to claim16, wherein: the first liquid crystal comprises at least one ofpolymerizable nematic monomers and oligomers the first liquid crystalbeing solidified by three-dimensional cross-linking; and the secondliquid crystal comprises at least one of polymerizable cholestericmonomers and oligomers, the second liquid crystal being solidified bythree-dimensional cross-linking.
 18. The process according to claim 16,wherein: the first liquid crystal comprises a nematic liquid crystallinepolymer, the first liquid crystal being solidified into a glassy stateby cooling; and the second liquid crystal comprises a cholesteric liquidcrystalline polymer, the second liquid crystal being solidified into aglassy state by cooling.
 19. The process according to claim 16, whereinapplying second liquid crystal comprises applying the second liquidcrystal in a coating thickness selected so that directors of liquidcrystalline molecules on the first surface of the C plate typeretardation layer and directors of liquid crystalline molecules on asecond surface of the C plate type retardation layer are substantiallyparallel.
 20. The process according to claim 16, further comprisingbringing a second alignment layer into contact with a surface of theapplied second liquid crystal opposite from the A plate type retardationlayer before solidifying the second liquid crystal so that, when thesecond liquid crystal is solidified, directions of directors of liquidcrystalline molecules on first surface of the C plate type retardationlayer and a second surface of the C plate type retardation layer areregulated.
 21. The process according to claim 16, further comprisingbringing a second alignment layer into contact with a surface of theapplied first liquid crystal opposite from a surface in contact with thealignment layer before solidifying the first liquid crystal so that,when the first liquid crystal is solidified, directions of directors ofliquid crystalline molecules on the first and second surfaces of the Aplate type retardation layer are regulated.